The invention relates to the field of pressure controllers for fluidic pumping systems and, in particular, to pressure controllers for intravenous blood pumps.
There are a number of medical treatments, such as ultrafiltration, apheresis and dialysis, that require blood to be temporarily withdrawn from a patient and returned to the body shortly thereafter. While the blood is temporarily outside of the body, it flows through an xe2x80x9cextracorporeal blood circuitxe2x80x9d of tubes, filters, pumps and/or other medical components. In some treatments, the blood flow is propelled by the patient""s blood pressure and gravity, and no artificial pump is required. In other treatments, blood pumps in the extracorporeal circuit provide additional force to move the blood through the circuit and to control the flow rate of blood through the circuit. These pumps may be peristaltic or roller pumps, which are easy to sterilize, are known to cause minimal clotting and damage to the blood cells, and are inexpensive and reliable.
Brushed and brushless DC motors are commonly used to rotate peristaltic pumps. A motor controller regulates the rotational speed of blood pumps. The speed of a pump, expressed as rotations per minute (RPM), regulates the flow rate of the blood through the circuit. Each revolution of the pump moves a known volume of blood through the circuit. Thus, the blood flow rate through the circuit can be easily derived from the pump speed. Accordingly, the pump speed provides a relatively accurate indicator for the volume flow of blood through an extracorporeal circuit.
Existing pump controllers may be as simple as a potentiometer that regulates the voltage to the pump DC motor. The pump speed is proportional to the voltage applied to the pump motor. By increasing the voltage, the speed of the pump increases and, similarly, the blood flow increases through the extracorporeal circuit. More sophisticated existing pump controllers, such as used in current dialysis machines, include a microprocessor that executes a software/firmware program to regulate the pump speed and, thus, blood flow in accordance with pump/flow settings entered by an operator. In these controllers, the microprocessor receives input commands from an operator who selects a desired blood flow using a user interface on the controller housing. The microprocessor determines the pump motor speed needed to provide the selected blood flow rate, and then issues commands to the pump motor to run at the proper speed.
To improve the accuracy and reliability of the blood flow through an extracorporeal circuit, existing pump controller microprocessors receive feedback signals from, for example, tachometers or optical encoders that sense the actual speed of the pump motor. Similarly, feedback signals have been provided by ultrasonic flow probes that measure the actual flow of blood in the extracorporeal circuit. By comparing the desired pump speed or flow rate to the measured pump speed or flow rate, the microprocessor can properly adjust the pump speed to correct for any difference between the desired and actual speed or rate. In addition, a calculated or measured flow value (actual flow rate) may be displayed on the pump console for viewing by the operator for a visual comparison with the desired flow setting.
The microprocessor controllers for blood pumps have, in the past, relied on both open and closed loop control of the motor speed. The open loop control normally consists of a constant feed forward voltage (based on the back EMF constant of the motor). The closed loop control systems use velocity feedback in the form of a tachometer, encoder or resolver to maintain constant pump flow. A constant flow control loop allows a user to set the blood flow rate, and the controller regulates the pump speed to maintain a constant blood flow, unless a malfunction occurs that would require the pump to be shut-down to protect the patient. The open loop control systems have the disadvantage that an increase or decrease in torque or motor resistance due to temperature will result in some variation in pump flow. This variation will generally be small when the motor is geared. Torque variations are not an issue for closed loop control systems because they use the motor velocity as feedback and adjust the supply current or voltage to the motor in order to maintain constant velocity.
Existing blood pump controllers include various alarms and interlocks that are set by a nurse or a medical technician (collectively referred to as the operator), and are intended to protect the patient. In a typical dialysis apparatus, the blood withdrawal and blood return pressures are measured in real time, so that sudden pressure changes are quickly detected. Sudden pressure changes in the blood circuit are treated as indicating an occlusion or a disconnect in the circuit. The detection of a sudden pressure change causes the controller to stop the pump and cease withdrawal of blood. The nurse or operator sets the alarm limits for the real time pressure measurements well beyond the expected normal operating pressure for the selected blood flow, but within a safe pressure operating range.
Examples of existing blood pump controllers are disclosed in U.S. Pat. Nos. 5,536,237 (""237 Patent) and 4,657,529 (""529 Patent). The controllers disclosed in these patents purport to optimize the rate of blood flow through a blood circuit based on a pressure vs. flow rate control curve. However, these patents do not teach controlling a blood pump based on control curves for both withdrawal and infusion pressures, and do not suggest reversing blood flow to relieve an occlusion. The authors of the ""237 and ""529 Patents acknowledged that during blood withdrawal, treatment is often interrupted if the vein is collapsed. They further acknowledged that as the result of such collapse the needle could be drawn into the blood vessel wall that makes the recovery difficult. The remedy proposed by these authors is to always withdraw blood at a rate that prevents the collapse of the vein by applying a complex system of identification of the vein capacity prior to treatment. The ""237 Patent specifically addresses the difficulty of generating ad-hoc pressure flow relationships for individual patients with low venous flow capacity. However, the experience of the present applicants is that the withdrawal properties of venous access in many patients are prone to frequent and often abrupt changes during treatment. Accordingly, the approach advanced in the ""237 and ""529 Patents of attempting to always avoid vein collapse will fail when a vein collapse condition occurs and does not provide any remedy for vein collapse (other than to terminate treatment and call a nurse or doctor).
Pressure conditions in a blood circuit often change because of blood viscosity changes (that in turn affect flow resistance), and because of small blood clots that form where blood stagnates on the surface of tubes and cannulae. These small clots partially occlude the blood circuit, but do not totally obstruct blood flow through the circuit. These clot restrictions increase the flow resistance and, thus, increase the magnitude of pressure in the circuit. Accordingly, an assumption that the flow resistance and pressure are constants in a blood circuit may not be valid. Gravity is another source of pressure change in a blood circuit. Gravity affects the pressure in a blood circuit. The pressure in the circuit will change due to gravity if the patient moves such that the height changes occur with respect to the vertical position of the circuit entry and exit points on the patient""s arms relative to the pressure sensors. The pressure sensors in the blood circuit may detect a change that is indicative of a change in the patient""s position, rather than a clot in the circuit. For example, if the patient sits up, pressure sensors will detect pressure changes of the blood in the circuit.
If the gravity induced pressure changes are sufficiently large, prior pump controllers tended to activate an alarm and shut down the blood pump. The operator then has to respond to the alarm, analyze the situation and remedy the malfunction or change the alarm limits if the flow path conditions have changed. In more advanced systems the operator sets the alarm window size (for example, plus or minus 50 mmHg about a mean point), and the machine will automatically adjust the mean point in proportion to the flow setting change. If the measured pressure exceeds the pressure range set by the window, the blood flow is automatically stopped. However, existing systems do not adjust the pump speed in response to changes in flow resistance, but rather, shut down if the resistance (and hence fluid pressure) become excessive.
A blood flow controller has been developed that controls blood flow through an extracorporeal blood circuit. The controller regulates the flow rate through the circuit such that: (a) blood is withdrawn from a peripheral vein in the patient (which veins are usually small, collapsible tubes) at a blood flow rate sustainable by the vein and which avoids collapsing the vein, and (b) blood pressure changes in the circuit are compensated for by adjusting pump speed and hence the flow rate through the circuit. The controller sets a flow rate of blood through the circuit based on both a maximum flow rate limit and a variable limit of pressure vs. flow rate. These limits may be preprogrammed in the controller and/or selected by an operator.
A novel blood withdrawal system has been developed that enables rapid and safe recovery from occlusions in a withdrawal vein without participation of an operator, loss of circuits to clotting, or annoying alarms. Applicants have developed a technique of compensating for and remedying temporary vein collapse when it occurs during blood withdrawal. They recognized that not all episodes of a vein collapse require intervention from a doctor or nurse, and do not require that blood withdrawal ceased for an extended period. For example, vein collapse can temporarily occur when the patient moves or a venous spasm causes the vein to collapse in a manner that is too rapid to anticipate and temporary. There has been a long-felt need for a control system for an extracorporeal circuit that can automatically recover from temporary occlusions. Applicants developed a system that temporarily stops blood withdrawal when vein collapse occurs and, in certain circumstances, infuses blood into the collapsed vein to reopen the collapsed vein. Applicants"" approach to vein collapse is counterintuitive and contrary to the approach disclosed in the ""237 and ""529 Patents.
Peripheral vein access presents unique problems that make it difficult for a blood withdrawal controller to maintain constant flow and not to create hazards for the patient. These problems are unlike those encountered with conventional dialysis treatments that rely on a surgically created arterio-venous shunt or fistula to withdraw blood and are administered in controlled dialysis centers. Using the present controller, for example, a patient may stand up during treatment and thereby increase the static pressure head height on the infusion side resulting in a false occlusion. The controller adjusts the blood flow rate through the extracorporeal circuit to accommodate for pressure changes. As the patient rises each centimeter (cm), the measured pressure in the extracorporeal circuit may increase by 0.73 mm Hg (milliliter of mercury). A change in height of 30 cm (approximately 1 ft) will result in a pressure change of 21 mm Hg. In addition, the patient may bend his/her arm during treatment and, thereby, reduce the blood flow to the withdrawal vein. As the flow through the withdrawal catheter decreases, the controller reduces pump speed to reduce the withdrawal pressure level. Moreover, the blood infusion side of the blood circulation circuit may involve similar pressure variances. These infusion side pressure changes are also monitored by the controller which may adjust the pump to accommodate such changes.
The controller may be incorporated into a blood withdrawal and infusion pressure control system which optimizes blood flow at or below a preset rate in accordance with a controller algorithm that is determined for each particular make or model of an extraction and infusion extracorporeal blood system. The controller is further a blood flow control system that uses real time pressure as a feedback signal that is applied to control the withdrawal and infusion pressures within flow rate and pressure limits that are determined in real time as a function of the flow withdrawn from peripheral vein access.
The controller may govern the pump speed based on control algorithms and in response to pressure signals from pressure sensors that detect pressures in the blood flow at various locations in the extracorporeal circuit. One example of a control algorithm is a linear relationship between a minimum withdrawal pressure and withdrawal blood flow. Another possible control algorithm is a maximum withdrawal flow rate. Similarly, a control algorithm may be specified for the infusion pressure of the blood returned to the patient. In operation, the controller seeks a maximum blood flow rate that satisfies the control algorithms by monitoring the blood pressure in the withdrawal tube (and optionally in the infusion tube) of the blood circuit, and by controlling the flow rate with a variable pump speed. The controller uses the highest anticipated resistance for the circuit and therefore does not adjust flow until this resistance has been exceeded. If the maximum flow rate results in a pressure level outside of the pressure limit for the existing flow rate, the controller responds by reducing the flow rate, such as by reducing the speed of a roller pump, until the pressure in the circuit is no greater than the minimum (or maximum for infusion) variable pressure limit. The controller automatically adjusts the pump speed to regulate the flow rate and the pressure in the circuit. In this manner, the controller maintains the blood pressure in the circuit within both the flow rate limit and the variable pressure limits that have been preprogrammed or entered in the controller.
In normal operation, the controller causes the pump to drive the blood through the extracorporeal circuit at a set maximum flow rate. In addition, the controller monitors the pressure to ensure that it conforms to the programmed variable pressure vs. flow limit. Each pressure vs. flow limit prescribes a minimum (or maximum) pressure in the withdrawal tube (or infusion tube) as a function of blood flow rate. If the blood pressure falls or rises beyond the pressure limit for a current flow rate, the controller adjusts the blood flow by reducing the pump speed. With the reduced blood flow, the pressure should rise in the withdrawal tube (or fall in the return infusion tube). The controller may continue to reduce the pump speed, until the pressure conforms to the pressure limit for the then current flow rate.
When the pressure of the adjusted blood flow, e.g. a reduced flow, is no less than (or no greater than) the pressure limit for that new flow rate (as determined by the variable pressure vs. flow condition), the controller maintains the pump speed and operation of the blood circuit at a constant rate. The controller may gradually advance the flow rate in response to an improved access condition, provided that the circuit remains in compliance with the maximum rate and the pressure vs. flow limit.
The controller has several advantages over the prior art including (without limitation): that the controller adjusts the pump speed to regulate the blood flow rate and maintain the blood pressure within prescribed limits, without requiring the attention of or adjustment by an operator; the controller adjusts blood flow in accordance with an occlusion pressure limit that varies with flow rate, and the controller adaptively responds to partial occlusions in the withdrawal blood flow. In addition, the controller implements other safety features, such as to detect the occurrence of total unrecoverable occlusions in the circuit and disconnections of the circuit, which can cause the controller to interpret that blood loss is occurring through the extracorporeal circuit to the external environment and stop the pump.
The controller may also compensate for variances in flow restrictions using control algorithms that apply two pressure targets: a withdrawal occlusion pressure target, and an infusion occlusion pressure target. By compensating for two control targets, the controller can monitor and discretely or simultaneously adjust for flow restrictions, e.g., partial occlusions, that occur in the withdrawal or infusion lines of the blood circuit. For example, if an occlusion occurs in the withdrawal vein, the pressure drop in the withdrawal line is sensed by the controller which in turn reduces the flow rate in accordance with an adjustable withdrawal pressure limit. The pump continues at a reduced speed, provided that the variable pressure vs. flow limit is satisfied and the boundary limits of the pressure vs. flow relationship are not exceeded. The controller may be required to stop the pump entirely, if the blood does not flow sufficiently (with acceptable pressure) at any pump speed. In such a condition, the controller will slow the pump to a stop as the withdrawal pressure target decreases. When the flow drops below a preprogrammed limit for a preprogrammed time, the pump will also be stopped by the flow controller and the user alarm will be activated.
In the event of a withdrawal pressure occlusion that terminates flow, the flow controller will temporarily reverse the flow of the pump in an attempt to remove the withdrawal occlusion. The occlusion algorithms are still active during this maneuver, and will also terminate flow if the occlusion cannot be removed. The volume displacement of the pump is limited to a specific number of revolutions to ensure that the pump does not infuse air into the patient.
With respect to the infusion pressure target, if a partial occlusion occurs in the infusion vein, the pressure controller detects a pressure rise in the return line and reduces the infusion rate. The controller will continue to reduce the pump speed and reduce the pressure in the infusion line. If the occlusion is total, the controller will quickly reduce the pump speed to a stop and avoid excessive pressure being applied to the infusion vein. The controller may also activate an alarm whenever the pump speed is stopped (or reduced to some lower speed level).
An application of the controller is in an extracorporeal blood system that includes a filter and blood pump in a blood circuit. This system extracts excess fluid from the blood of a fluid overloaded patient. The amount of excess fluid that is withdrawn from the blood is set by the operator at a clinically relevant rate to relieve the overloaded condition of the patient. The system accesses the peripheral veins of the patient, and avoids the need to access central venous blood. The filter system provides a simple method for controlling blood extraction from a peripheral vein. Thus, the blood filtering system could be used in a physician""s clinic and outside of an ICU (intensive care unit of a hospital).
The blood filter system may provide an acceptable level of invasiveness, e.g., minimally invasive, for a fluid removal treatment in the desired environment and patient population via a peripheral vein preferably in an arm of a patient. Such access is commonly established by a nurse in order to draw blood or to infuse drugs. The filter system may withdraw up to 80% or intermittently up to 100% of the available blood fluid flow from the vein without causing the vein to collapse. Because of its reduced invasiveness, the filter system does not require an ICU or a special dialysis setting to be administered to a patient. If an apparatus for slow continuous ultrafiltration was available that would draw and re-infuse blood into the body using the access site similar to a common intravenous (IV) therapy, such a device would have a widespread clinical use.
The controller/filter system further provides a mechanism for maintaining patient safety while preventing false alarms when blood pressure measurements are made with or without the use of a blood pressure cuff.